For an industrial undertaking it is advantageous to recycle the liquor resulting from fermentations with a view to using it in a new fermentation medium. Among other things, this makes it possible to reduce the amounts of water entering and leaving the fermenters.
Traditionally, the liquor is recycled after fermentation on media containing sources of sugars obtained from beet or cane, such as LS2 media (Lean Syrup from the 2nd crystallization of sugar), but also after fermentation on media containing sugars derived from wheat or maize. The recycled liquor can supply nutritional elements for the yeast, but in particular it has a certain toxicity for the yeast, and this toxicity will increase as the degree of recycling increases.
Moreover, certain raw materials used in the fermentation media are in themselves toxic to yeast. This is the case for example with molasses (also called LS3, or Lean Syrup from the 3rd crystallization of sugar).
The toxicity of the recycled liquor and of these toxic raw materials results notably from the presence of organic acids and salts.
In a toxic medium containing, partially or completely, recycled liquor and/or toxic raw materials, the known yeasts used in alcoholic fermentation have a kinetics of alcohol production slower than in a non-toxic medium. Moreover, the maximum amount of alcohol produced by the known yeasts in fermentation in a toxic medium is lower than in a non-toxic medium.
When the raw materials used are toxic in themselves, dilution of them in the fermentation medium makes it possible to limit their toxic effect. However, this dilution causes very large additional costs, connected notably with lower volume-based productivity and higher costs of cooling and distillation.
Attempts to solve the problem of toxic media have until now focused on making the fermentation medium less toxic. However, no really satisfactory solution has been found for this problem.
There is therefore a real need to provide an alternative solution for improving alcohol production by yeasts in toxic fermentation media.
“Improvement of alcohol production” means an improvement of the kinetics of alcohol production in a toxic fermentation medium, while maintaining an acceptable quantity of alcohol produced.